Project Summary/Abstract Triple-negative breast cancer (TNBC) remains an ongoing clinical and therapeutic challenge. Combining the absence of a targeted therapy and the higher risk of metastasis and relapse, patients diagnosed with TNBC face a very poor prognosis. The aggressive nature of this breast cancer sub-type has been attributed to the presence of breast cancer stem cells (BCSCs) that possess the ability to self-renew and initiate tumor growth, which are key drivers of metastases. The Janus kinase/signal transducer and activator of transcription (JAK/STAT3) have been shown to not only be critical nodes of this feature, but also overactive in TNBC. Targeting of this pathway using inhibitors, either small-molecule drugs or siRNA, represents a promising treatment method. However, delivery of these therapeutics requires the use of a drug delivery system that is capable of overcoming the biological barriers posed by the body. To this aim, nanoparticles have been widely explored as drug delivery vehicles that carry cancer therapeutics to the tumor site. However, the success of these platforms has been limited due to their rapid clearance by the immune system and non-specific accumulation in other organs. This study aims to utilize a novel biomimetic leukocyte- based nanoparticle platform (i.e. the leukosome) as a drug delivery vehicle to carry therapeutics that target the JAK/STAT3 pathway in TNBC while overcoming the aforementioned limitations of previously utilized nanoparticle systems. Leukosomes loaded with inhibitors of this pathway will be synthesized and characterized, followed by in vitro tests to determine the dose-response cell toxicity of drug-loaded particles, their internalization and intracellular trafficking in TNBC cell lines and validate their suppression of the JAK/STAT3 pathway. Finally, a patient-derived xenograft model of TNBC will be utilized to determine the in vivo biodistribution of the leukosomes and evaluate their therapeutic effects on tumor growth and the targeting of the BCSC population. By combining the features of cell and cancer biology with the engineering of fine-tuned nanoparticles, this platform will provide a novel solution to a currently unmet clinical need in the treatment of TNBC.